This invention relates to multiple pole coupling discs of the type used in an electromagnetic coupling such as an electromagnetic clutch or brake. The coupling disc may be part of a rotary or non-rotary field or may be a rotary or non-rotary armature.
A typical electromagnetic coupling is disclosed in Silvestrini et al U.S. Pat. No. 4,187,939 and, in that particular case, the coupling is an electromagnetic clutch having a rotary armature disc made of magnetic material such as steel and having a field with a rotary coupling disc or rotor which also is made of magnetic material. When the coil of the field is excited, magnetic flux threads a path between the rotor and the axially opposing armature and attracts the armature into engagement with the working face of the rotor to couple the two for rotation in unison.
In the coupling disclosed in the Silvestrini et al patent, the armature is formed with a ring of angularly spaced "banana" slots while the rotor is formed with two concentric rings of angularly spaced banana slots located on opposite sides of the ring of slots in the armature. The banana slots form high reluctance air gaps causing the rotor and armature to define four magnetic poles which increase the torque of a coupling having a coil of a given diameter. By forming an additional ring of slots in each of the rotor and armature, the coupling may be constructed as a six-pole coupling with even higher torque capacity.
Until just recently, the banana slots conventionally have been stamped in the rotor and armature. Presently available stamping techniques dictate that, as a general rule, the radial width of the slots cannot be substantially less than approximately 3/4 the thickness of the disc. As a result, difficulty is encountered in stamping multiple rings of slots in a comparatively thick disc which is relatively small in diameter. In addition, stamping of the slots leaves burrs at the edges of the slots and tends to impose restrictions on the location of the slots in the disc and on the shape of the slots. It is difficult to maintain concentricity between adjacent rows of slots and it is difficult to keep all portions of the disc of a uniform thickness. The design of the disc thus tends to be dictated by tooling considerations rather than magnetic characteristics.
As an alternative to slotting the rotor and armature to form high reluctance air gaps, channels may be machined in the disc and then filled with nonmagnetic material to define high reluctance barriers between the poles. Subsequently, the disc is machined to remove the bottoms of the magnetic channels and eliminate the flux leakage paths which otherwise would be created across the bottoms of the channels. This manufacturing process is relatively expensive and becomes even more so when each disc is formed with two or more high reluctance rings.
Formation of the slots in a coupling disc through the use of a laser beam is disclosed in commonly assigned Booth et al U.S. Pat. No. 4,685,202. In the method disclosed in that patent, the laser beam forms continuous slots which are immediately backfilled with non-magnetic material. Alternatively, the method contemplates the formation of angularly spaced banana slots separated by non-magnetic bridges which are formed by backfilling the spaces between the slots with non-magnetic material.
The methods disclosed in the aforementioned Booth et al patent represent remarkable improvements in the art of magnetic coupling discs. Even those methods, however, have some limitations. For example, the formation of slots of any substantial radial width requires the use of a very powerful laser having a beam of substantial diameter. In addition, backfilling of the slots or portions thereof imposes some restriction on the cross-sectional shape and/or the orientation of the slots.
Booth et al U.S. Pat. No. 4,818,840 discloses another method of forming slots in an electromagnetic coupling disc through use of a laser. Specifically, the laser beam traces around the perimeter of each slot to be formed and forms the slot by cutting a slug of material from the disc. This method enables relatively precise control of the shape, location and edge finish of the slots but is somewhat slow from a manufacturing standpoint since the entire perimeter of each slot must be traced by the laser beam. In addition, it is necessary to reprogram the path of travel of the laser beam each time the slot configuration, location or size is changed.